Recombinant Brucella melitensis biotype 1 Flagellar M-ring protein (fliF)

What is the Flagellar M-ring Protein (FliF) and what is its role in Brucella melitensis?

FliF is a structural protein that forms the MS (Motor Switch) ring of the bacterial flagellar apparatus, which serves as the base for the flagellar structure. In B. melitensis, despite being traditionally classified as non-motile, genome sequencing has revealed the presence of all necessary flagellar genes (except for chemotactic systems) to assemble a functional flagellum . The FliF protein (encoded by gene BMEII0151) is a critical component of this structure and forms the foundation upon which other flagellar components are assembled . Electron microscopy studies have confirmed that B. melitensis produces a polar and sheathed flagellar structure visible during early logarithmic growth phase .

How is the fliF gene regulated in Brucella melitensis?

The expression of fliF in B. melitensis is growth phase-dependent. Microarray analysis has shown that fliF (BMEII0151) is differentially expressed in late-log phase cultures compared to stationary phase cultures . Additionally, quorum-sensing (QS) genes play a significant role in regulating flagellar gene expression. The transcriptional regulator VjbR is required for the expression of various flagellar genes including fliF in B. melitensis . This regulation contributes to the transient expression of the flagellum during specific growth phases and may be related to the invasion process during infection .

What methods are commonly used to express recombinant FliF protein?

Recombinant FliF protein can be expressed using standard recombinant DNA technology approaches. The general methodology includes:

• PCR amplification of the fliF gene from B. melitensis genomic DNA

• Cloning into expression vectors (commonly pET vectors) with appropriate restriction enzymes

• Expression in E. coli systems (typically E. coli BL21 or ER2566 strains)

• Purification using affinity chromatography with His-tag systems

The detailed protocol based on similar flagellar protein expression studies involves:

• Gene amplification using primers that incorporate restriction enzyme sites

• Ligation into expression vectors

• Transformation into competent E. coli cells

• Induction of protein expression using IPTG

• Cell lysis and purification of the recombinant protein

What is known about the immunogenicity of flagellar proteins in Brucella species?

Flagellar proteins have demonstrated significant immunogenic properties in Brucella species. Studies have shown that:

• Recombinant flagellar proteins can induce protective immune responses in mice models

• Immunization with flagellar proteins results in both humoral and cell-mediated immune responses

• Specifically, flagellar proteins FlgJ and FliN have been shown to induce protection against B. abortus infection

When mice were immunized with these proteins, they developed:

• Vigorous immunoglobulin G (IgG) responses

• Enhanced secretion of IFN-γ from splenocytes upon in vitro stimulation

• Significant protection against bacterial challenge

How does mutation in the fliF gene affect Brucella melitensis virulence and pathogenicity?

Mutation studies have revealed critical insights into FliF's role in Brucella pathogenesis. Research shows that fliF mutants exhibit:

• Impaired intracellular survival and replication within host cells

• Inability to establish chronic infection in murine models

• Altered trafficking within macrophages

In experimental infections, while wild-type B. melitensis could establish persistent infection, fliF mutants were unable to maintain chronic infection when administered via intraperitoneal route in mice . This suggests that the FliF protein plays a crucial role in the bacteria's ability to persist in the host, which is a hallmark of brucellosis.

Interestingly, while flagellar mutants show reduced persistence in vivo, they often don't display discernible phenotypes in cellular models of infection, suggesting that the flagellum's role may be more significant in the context of the complete host immune environment rather than in isolated cell culture systems .

What are the methodological approaches for constructing fliF gene knockout mutants in Brucella?

Creating fliF knockout mutants requires precise genetic manipulation. Based on similar Brucella flagellar gene deletion studies, the methodology typically involves:

• Lambda Red Recombination System:

  • Transform B. melitensis with recombinant plasmid pSIM7

  • Induce expression of the λ red recombinase at 42°C

  • Replace the target gene with a kanamycin resistance cassette amplified from plasmid pKD4

  • Select transformants on kanamycin-supplemented media

  • Confirm deletion by PCR verification

• Complementation Studies:

  • Amplify the wild-type fliF gene from B. melitensis genomic DNA

  • Clone into a complementation vector (such as pVB1)

  • Transform the construct into the knockout strain

  • Confirm restoration of function through phenotypic and functional assays

This approach allows researchers to definitively attribute phenotypic changes to the specific gene deletion and confirm findings through complementation studies.

How does the expression of FliF differ between various Brucella species and biovars?

Comparative genomic analyses have revealed nuances in flagellar gene expression across Brucella species and biovars:

• B. melitensis expresses a polar sheathed flagellum visible by transmission electron microscopy during early logarithmic growth phase

• B. abortus contains flagellar genes but their expression patterns differ from B. melitensis

• Expression is highly influenced by growth conditions and environmental factors

Whole genome phylogeny studies of B. melitensis isolates (n=355) have revealed genomic diversity even within the same species, with Indian isolates clustering primarily in the East Mediterranean lineage . This genetic diversity may influence flagellar gene expression and function across different strains and geographic regions.

What is the potential of recombinant FliF as a vaccine candidate against brucellosis?

Research on flagellar proteins as vaccine candidates has shown promising results:

While specific studies on FliF as a vaccine candidate are still emerging, related flagellar proteins have demonstrated:

• Ability to stimulate both humoral and cell-mediated immune responses

• Protection against bacterial challenge in mouse models

• Potential advantages over live attenuated vaccines in terms of safety

The subunit vaccine approach using recombinant flagellar proteins offers several advantages:

• Eliminates risks associated with live attenuated vaccines

• Allows for precise control of antigenic components

• Enables targeted immune responses against specific epitopes

How can advanced proteomics approaches be used to study FliF interactions with other flagellar proteins?

Advanced proteomics offers powerful tools for investigating FliF's interactions:

• Protein-Protein Interaction Studies:

  • Co-immunoprecipitation with anti-FliF antibodies

  • Yeast two-hybrid screening

  • Crosslinking mass spectrometry to capture interaction networks

  • Blue native PAGE for protein complex analysis

• Structural Analysis:

  • X-ray crystallography of purified recombinant FliF

  • Cryo-electron microscopy of flagellar complexes

  • Hydrogen-deuterium exchange mass spectrometry for dynamic interaction mapping

• Interaction Verification:

  • Surface plasmon resonance to quantify binding affinities

  • Fluorescence resonance energy transfer (FRET) for in vivo interaction studies

  • Bacterial two-hybrid systems for validation

These approaches can reveal how FliF interacts with other flagellar proteins such as FlgJ, which has been shown to be important for Brucella virulence and flagellar function .

What are the experimental approaches to evaluate the immunogenicity of recombinant FliF protein?

Comprehensive evaluation of FliF immunogenicity requires multi-faceted approaches:

• Humoral Immune Response Assessment:

  • ELISA to measure specific IgG, IgG1, and IgG2a antibody titers

  • Western blotting to confirm antibody specificity

  • Immunoprecipitation to evaluate antibody-antigen interactions

• Cellular Immune Response Evaluation:

  • Cytokine profiling (IFN-γ, IL-2, IL-4, TNF-α) by ELISA or cytometric bead array

  • T-cell proliferation assays using CFSE labeling

  • ELISpot assays to enumerate cytokine-producing cells

  • Real-time RT-PCR for cytokine gene expression analysis

• In vivo Protection Studies:

  • Challenge with virulent Brucella strains following immunization

  • Bacterial load determination in spleen and other organs

  • T-cell subset depletion studies to determine protective mechanisms

Studies with other Brucella outer membrane proteins have shown that immunization can elicit strong Th1 responses, with CD4+ T cells secreting IL-2 and IFN-γ, while CD8+ T cells induce cytotoxic T-lymphocyte activity against Brucella-infected macrophages .

How does the regulation of FliF expression in Brucella differ from other alphaproteobacteria?

The regulation of flagellar proteins in Brucella follows a unique pattern compared to other alphaproteobacteria:

• In Caulobacter crescentus and Sinorhizobium meliloti, flagellar gene expression follows a hierarchical cascade

• In contrast, Brucella melitensis employs a different regulatory model as demonstrated by Ferooz et al.

• The production of flagellar components in Brucella is not strictly dependent on the completion of prior structures

Specifically for Brucella:

• The FlaF and FlbT regulators have opposite effects on flagellin production

• The alternative sigma factor RpoE1 represses production of hook protein (FlgE) and flagellin (FliC) via activation of an unknown repressor

• This repressor negatively regulates the expression of the master flagellar regulator FtcR

This distinctive regulatory model may have important implications for understanding how FliF expression is controlled during different phases of the Brucella life cycle and infection process.

What techniques can be used to detect and differentiate recombinant FliF protein from native FliF in experimental settings?

Several techniques can effectively distinguish recombinant from native FliF protein:

• Immunological Methods:

  • Use of epitope tag-specific antibodies (His-tag, FLAG-tag) that recognize only the recombinant protein

  • Development of monoclonal antibodies against unique epitopes in the recombinant construct

  • Two-dimensional gel electrophoresis followed by western blotting

• Mass Spectrometry Approaches:

  • Peptide mass fingerprinting to identify tag-derived peptides

  • Multiple reaction monitoring (MRM) to target peptides unique to the recombinant protein

  • Top-down proteomics for intact protein analysis

• Size-Based Differentiation:

  • SDS-PAGE to detect size differences due to tags or fusion partners

  • Native PAGE to assess differences in oligomeric state or folding

  • Size-exclusion chromatography for purification and characterization

These approaches are particularly important when evaluating the expression of FliF in complementation studies of knockout mutants or when assessing the immunological properties of the recombinant protein in vaccine development .